Various types of electrically operated devices available today utilize solid-state circuit components. Such solid-state components are designed to be operated within specified voltage limits and are subject to damage when voltages outside of the specified range are applied.
An example of such an electrical circuit is the motor circuit associated with an electrical vehicle, for example, an industrial lift truck. Such motor circuits often include an electric motor connected across a direct current power supply, for example, a storage battery. Power is typically supplied to the motor through a solid-state current chopping device, such as a transistor or thyristor. In addition to the motor, various other electrical devices are frequently connected across the power supply.
Although the nominal voltage delivered from the direct current power supply is substantially constant, operation of the various other connected devices can cause the actual voltage of the power supply to vary about the nominal magnitude. Such transient variation in power supply voltage are commonly referred to as "ripple", and are particularly undesirable when such ripple or transient voltages exceed the voltage limits of the associated motor circuit solid state components.
Recognizing that damage can result from excessive power supply transient voltages, manufacturers have devised methods of maintaining the peak transient voltage within safe limits. Most common is the use of a head capacitor connected across the power supply in parallel with the associated components. The value of the head capacitor is determined by the circuit in question, and is such that the head capacitor is able to resist instantaneous changes in the nominal power supply voltage, or, at least, is able to maintain the magnitude of such changes within prescribed bounds.
The use of a head capacitor has been found to satisfactorily counter the transient voltage problem. However, head capacitors often fail when used in an industrial environment. The capacitors are subjected to extremes of temperature and vibration, and often either fail catastrophically or gradually change in essential characteristics over a period of time. Such failure allows transient voltages to exceed the safe limits established, and often results in catastrophic failure of the various solid state components associated with the motor circuit.
The present invention is directed to overcoming one or more of the problems as set forth above.